Reflective liquid crystal light valve

ABSTRACT

Reflective liquid crystal light valves are disclosed. A liquid crystal cell is also disclosed comprising a transparent electrode, a reflective electrode, and a twisted nematic liquid crystal layer interposed therebetween. A first alignment layer with a first alignment direction disposed on the transparent electrode. A second alignment layer with a second alignment direction disposed on the reflective electrode, wherein a first included angle φ is between the first and second alignment directions. A polarizing device is disposed on the exterior of the transparent electrode to provide an incident beam having a polarization direction, wherein a second included angle β is between the first alignment direction and the polarization direction. A relationship between the first included angle φ and the second included angle β satisfies φ/2&lt;β&lt;φ/2+30° or 90°+φ/2&lt;β&lt;φ/2+120°.

FIELD OF THE INVENTION

The invention relates to projection displays, and more particularly, toa reflective liquid crystal light valve for same.

BACKGROUND OF THE INVENTION

A reflective liquid crystal light valve is an important element in aprojection display. Reflective liquid crystal light valves typicallycomprise a polarizing beam splitter (PBS) and a reflective liquidcrystal cell. The size of each pixel of a high resolution projectiondisplay is approximately equal to a cell gap of the reflective liquidcrystal cell. As such, the fringe field between adjacent pixels caninterfere with and reorient the liquid crystal orientation and thendegrade image contrast and reduce display brightness. Therefore, todecrease the fringe field effect, a low driving voltage is used toachieve high resolution, high contrast ratio, and high brightness in theprojection display.

U.S. Pat. No. 5,490,003 to Sprang, the entirety of which is herebyincorporated by reference, discloses a reflective liquid crystaldisplay. The reflective liquid crystal display comprises a layer ofpositive dielectric anisotropic liquid crystal molecules with a twistangle and a polarizer having a polarization direction at the bisector ofthe twist angle.

U.S. Pat. No. 5,936,697 to Yang, the entirety of which is herebyincorporated by reference, discloses a self-compensated twisted nematic(SCTN) mode reflective light valve. The reflective light valve comprisesa SCTN mode reflective liquid crystal cell with negative dielectricanisotropic liquid crystal (LC) molecules, and a polarizer having apolarization direction at the bisector of the twist angle.

The conventional reflective light valve utilizing the bisector of thetwist angle, however, does not take boundary layer residual phaseretardation into consideration. Thus, in practice, the bisector is notin the proper polarization direction for achieving low operating voltageand high contrast ratio.

SUMMARY

According to various embodiments reflective liquid crystal light valveswith a predetermined polarization direction are provided. An exemplaryembodiment of a reflective liquid crystal light valve comprises a liquidcrystal cell comprising a transparent electrode disposed opposite areflective electrode with a twisted nematic (TN) mode liquid crystallayer interposed therebetween. The light valve can also include a firstalignment layer with a first alignment direction disposed on thetransparent electrode. A second alignment layer with a second alignmentdirection is disposed on the reflective electrode, wherein a firstincluded angle φ is between the first and second alignment directions. Apolarizer is disposed on the exterior of the transparent electrode toprovide an incident beam having a polarization direction, wherein asecond included angle β is between the first alignment direction and thepolarization direction. A relationship between the first included angleφ and the second included angle β can satisfy φ/2<β<φ/2+30° or90°+φ/2<β<φ/2+120°.

According to various embodiments the optimal polarization direction ofthe incident beam that provides improved results is not the bisectordirection between the first and second alignment directions. Therelationship between the first included angle φ and the second includedangle β can satisfy φ/2<β<φ/2+30° or 90°+φ/2<β<φ/2+120°. The reflectiveliquid crystal light valve can thus potentially achieve lower drivingvoltage and higher contrast ratio, improving display quality.

DESCRIPTION OF THE DRAWINGS

Reflective liquid crystal light valves can be more fully understood byreading the subsequent detailed description in conjunction with theexamples and references made to the accompanying drawings, wherein:

FIG. 1A depicts an operating principle using a reflective liquid crystalcell for an embodiment of a reflective light valve, which includes anincident beam being linear polarized;

FIG. 1B depicts a schematically sectional view of the reflective liquidcrystal cell shown in FIG. 1A according to various embodiments of theinvention;

FIG. 2 schematically depicts a relationship between the polarizationdirection of the incident beam and the alignment directions according toan embodiment of a light valve;

FIG. 3A depicts a graphical plot of the relationship between theazimuthal angle of eigenmode 1 and the residual retardation for anembodiment of a left-handedness 60°-TN liquid crystal cell atuniform-twist and two-layer models according to various embodiments ofthe invention;

FIG. 3B depicts a graphical plot of the relationship between theazimuthal angle of eigenmode 2 and the residual retardation for anembodiment of a left-handedness 60°-TN liquid crystal cell atuniform-twist and two-layer models according to various embodiments ofthe invention;

FIG. 4A depicts a local enlarged view of FIG. 3A;

FIG. 4B depicts a local enlarged view of FIG. 3B;

FIG. 5 depicts a graphical plot of the relationship between the residualretardation and the applied voltage for an embodiment of a 60°-TN liquidcrystal cell with a retardation value (dΔn) of 350 nm according tovarious embodiments of the invention;

FIG. 6A depicts a graphical plot of the relationship between thenormalized reflectance and the applied voltage for an embodiment of a60°-TN liquid crystal cell according to a first test;

FIG. 6B depicts a local enlarged view of FIG. 6A in the dark state;

FIG. 7A depicts a graphical plot of the relationship between thenormalized reflectance and the applied voltage for an embodiment of a57°-TN liquid crystal cell according to a second test;

FIG. 7B depicts a local enlarged view of FIG. 7A in the dark state;

FIG. 8A depicts a graphical plot of the relationship between thenormalized reflectance and the applied voltage for an embodiment of a55°-TN liquid crystal cell according to a third test;

FIG. 8B depicts a local enlarged view of FIG. 8A in the dark state;

FIG. 9A depicts a graphical plot of the relationship between thenormalized reflectance and the applied voltage for an embodiment of a50°-TN liquid crystal cell according to a fourth test;

FIG. 9B depicts a local enlarged view of FIG. 9A in the dark state;

FIG. 10A depicts a graphical plot of the relationship between thenormalized reflectance and the applied voltage for an embodiment of a45°-TN liquid crystal cell according to a fifth test;

FIG. 10B depicts a local enlarged view of FIG. 10A in the dark state;

FIG. 11A depicts a graphical plot of the relationship between thenormalized reflectance and the applied voltage for an embodiment of a40°-TN liquid crystal cell according to a sixth test;

FIG. 11B depicts a local enlarged view of FIG. 11A in the dark state;

FIG. 12A depicts a graphical plot of the relationship between thenormalized reflectance and the applied voltage for an embodiment of a65°-TN liquid crystal cell according to a seventh test;

FIG. 12B depicts a local enlarged view of FIG. 12A in the dark state;

FIG. 13A depicts a graphical plot of the relationship between thenormalized reflectance and the applied voltage for an embodiment of a70°-TN liquid crystal cell according to an eighth test;

FIG. 13B depicts a local enlarged view of FIG. 13A in the dark state;

FIG. 14 depicts a schematic diagram illustrating an embodiment of aprojection display apparatus, incorporating a controller according tovarious embodiments of the invention; and

FIG. 15 depicts a schematic diagram illustrating an electronic deviceincorporating an embodiment of a projection display apparatus accordingto various embodiments of the invention.

DETAILED DESCRIPTION

Reflective liquid crystal light valves according to various embodimentsare provided. An exemplary embodiment of a reflective light valve 90,shown in FIG. 1A, comprises a reflective liquid crystal display (e.g. areflective TN type liquid crystal cell 100) and a polarizing device(e.g. a polarizing beam splitter 7). The reflective light valve 90 iswell suited for the projection display. A representative projectiondisplay is illustrated, but is not intended to limit the disclosure.

The operating principles according to various embodiments of thereflective liquid crystal light valve 90 are illustrated in FIG. 1A. Anon-polarized incident light beam 6 from a light source becomes alinearly-polarized light 8 after passing through a polarizing device 7,such as a beam splitter (PBS), and being reflected 90° thereby, definingpolarized light 8 as p-wave 8. It is to be understood that otherpolarizing devices known in the art may also be used. Thelinearly-polarized light 8 then impinges on a reflective TN type liquidcrystal cell 100. As shown in FIG. 1A, the TN type liquid crystal cell100 comprises a transparent front panel 1 disposed opposite a reflectiverear panel 2 with a TN type liquid crystal material 5 interposedtherebetween.

FIG. 1B schematically depicts a sectional view of a TN type liquidcrystal cell, such as that labeled 100 in FIG. 1A. The front panel 1comprises a transparent substrate 11, a transparent electrode 12, and afirst alignment layer 13 with a first alignment direction 3 (shown inFIG. 2). The transparent substrate 11 can be glass. The transparentelectrode 12, such as indium tin oxide (ITO) or indium zinc oxide (IZO),is formed on the interior of the transparent substrate 11. The firstalignment layer 13 can be formed on the transparent electrode 12. Therear panel 2 comprises a substrate 21, a reflective electrode 22, and asecond alignment layer 23 with a second alignment direction 4 (alsoshown in FIG. 2). The substrate 21 can be a silicon wafer or any othersuitable semiconductor material. The reflective electrode 22, such as,for example, aluminum or silver, is formed on the substrate 21. Thesecond alignment layer 23 is formed on the reflective electrode 22. Theliquid crystal material 5 is disposed between the first and secondalignment layers 13 and 23, respectively. The liquid crystal material 5can comprise positive dielectric anisotropic (Δε>0) liquid crystalmolecules. The liquid crystal molecules, near the alignment layers 13and 23, are arranged along the alignment directions 3 and 4 shown inFIG. 2.

According to various embodiments, as depicted, for example in FIG. 1A,the TN type liquid crystal cell 100 is designed such that at or below acertain predetermined voltage defined as a threshold voltage, applied tothe two electrodes 12 and 22, the incident polarized light 8 can becomean s-wave 9 (or nearly s-wave) upon reflection from the liquid crystalcell 100. The s-wave 9 is a linearly polarized light with a direction ofpolarization perpendicular to that of the p-wave 8. The s-wave 9 iscapable of passing directly through the PBS 7 to serve as a projectionbeam 10. The projection beam 10 is then corrected by projection lenses(not shown) for projection onto a screen (not shown) for viewing. Thissituation represents the bright state of the projection display.

When an external voltage is applied across the two electrodes 12 and 22of the liquid crystal cell 100 at or above a certain voltage, defined asthe saturation voltage, the liquid crystal cell 100 behaves as anoptically isotropic medium. In this case, the impinging linearlypolarized light 8 will be reflected from the reflective liquid crystalcell 100, preserving the same direction of polarization (a p-wave inthis case). The reflected p-wave cannot directly pass through the PBS 7and will propagate backward opposite the incident beam 6. That is, thereflected p-wave does not project onto a screen (not shown) for viewing.This situation represents the dark state of the projection display. Inorder to get a high contrast ratio, a perfect dark state is desired. Assuch, the polarization state of the incident polarized beam 8 should bean eigenmode for the reflective liquid crystal cell 100 in order toobtain the desired contrast.

For better understanding, two different models (i.e. a uniform-twistmodel and a two-layer model) are provided to illustrate the eigenmode ofthe reflective TN type liquid crystal cell 100. According to variousembodiments, positive dielectric anisotropic (Δε>0) liquid crystalmolecules are utilized in the liquid crystal cell 100, and the pre-tiltangle at the substrate boundary is small (3˜5°). The liquid crystalmolecules undergo a uniform twist throughout the liquid crystal cell 100when the applied voltage is below a threshold voltage. When the appliedvoltage is around two times higher than the threshold voltage, theliquid crystal molecules in the middle of the liquid crystal cell 100are aligned almost parallel to the electric field between the panels 1and 2. However, the boundary layers of molecules near the front and rearsubstrate interfaces can be poorly distributed due to strong surfaceanchoring. Therefore, the TN type liquid crystal can be defined as auniform-twist model when the applied voltage is below the thresholdvoltage and as a two-layer model when the applied voltage is about twotimes higher than the threshold voltage.

In the uniform-twist model, there are two eigenmodes for the TN typeliquid crystal cell. Both eigenmodes are linearly polarized andorthogonal. In the mentioned eigenmodes, the azimuthal angles of linearpolarization are determined by “θ” in the following equation (1):$\begin{matrix}{{\tan\quad\theta} = {- \frac{{\cos\quad X} \pm \sqrt{1 - \left( \frac{{\Gamma sin}\quad X}{2X} \right)^{2}}}{\varphi\frac{\sin\quad X}{X}}}} & (1)\end{matrix}$

In the above equation, Γ=2πdΔn/λ and X={square root}{square root over(φ²+(Γ/2)²)}, wherein Γ is the phase of uniformly twisted TN type liquidcrystal molecules, d is the cell gap between two substrates 1 and 2, Δnis the birefringence of the liquid crystal material, λ is the wavelengthof the incident beam, and φ is the twist angle of the liquid crystalmolecules (i.e. the included angle between the first and secondalignment directions 3 and 4). Here, the left-handedness twist angle(for example, counterclockwise direction) is defined to be positive andthe right-handedness (for example, clockwise direction) twist angle asnegative. Referring to FIG. 2, the positive included angle φ is betweenthe first and second alignment directions 3 and 4. Numeral 25 denotesthe bisector of the included angle φ.

In the two-layer model, each boundary layer is referred to as anon-twisted uniaxial layer with residual phase ψ=2πα/λ, wherein α is theretardation of each boundary layer. Retardation a decreases as theapplied voltage increases. Similarly, there are two eigenmodes for thereflective TN type liquid crystal cell using the two-layer model. Bothof the mentioned eigenmodes are linearly polarized and orthogonal. Inthe mentioned eigenmodes, the azimuthal angles of linear polarizationare determined by “θ” in the following equation (2): $\begin{matrix}{{\tan\quad\theta} = {- \frac{{\cos\quad{\varphi cos}\quad X} \pm \sqrt{{\cos^{2}{\varphi cos}^{2}\psi} + {\sin^{2}\varphi}}}{\sin\quad\varphi}}} & (2)\end{matrix}$

When an intermediate voltage (the applied voltage between the thresholdvoltage and two times thereof) is applied, no approximation is madebecause of more complicated cases. Nevertheless, the azimuthal angles ofthe eigenmodes should be between the uniform-twist and two-layer models.

FIG. 3A is a graphical plot of the relationship between the azimuthalangle of eigenmode 1 and the residual retardation for an embodiment of aleft-handedness 60°-TN (i.e. twist angle φ is 60°) liquid crystal cellat uniform-twist and two-layer models according to various embodiments.FIG. 4A is a local enlarged view of FIG. 3A. FIG. 3B is a graphical plotof the relationship between the azimuthal angle of eigenmode 2 and theresidual retardation for the left-handedness 60°-TN liquid crystal cellin the uniform-twist and two-layer models according to variousembodiments. FIG. 4B is a local enlarged view of FIG. 3B. Referring toFIGS. 3A, 3B, 4A and 4B, the azimuthal angles of eigenmodes 1 and 2gradually reach the bisector 25 (i.e. φ/2=30°) of the twist angle orperpendicular to the bisector 25 (i.e. 90°+φ/2=120°). This is the reasonthat the cited references (U.S. Pat. Nos. 5,490,003 and 5,936,697)employ the bisector effect to achieve a dark state in simulation.

The bisector used in the conventional technology, however, does notachieve low operating voltages and/or high contrast ratios because ofthe poor polarization direction for achieving low operating voltage andhigh contrast ratio in practice. According to the conventionaltechnology, even when the applied voltage reaches three times thethreshold voltage, the residual retardation is still much greater than0. As a result, the azimuthal angles of the two eigenmodes are notexactly parallel to the bisector or perpendicular to the bisector. Onereason for the poor result is that the conventional technology does nottake boundary layer residual phase retardation into consideration.

Various tests were preformed and the parameters of the liquid crystalmolecules used in the tests of the specification are listed in Table 1.TABLE 1 Parameter of LC molecules Value Refractive index n_(e) 1.65Refractive index n_(o) 1.55 Ferroelectric index ε_(p) 12.0 Ferroelectricindex ε_(v) 4.0 Coefficient of elasticity k₁₁ 11.5E−12N Coefficient ofelasticity k₂₂  6.5E−12N Coefficient of elasticity k₃₃ 16.0E−12NPre-tilt angle 3°

In one test, the results of which are shown in FIG. 5, the residualretardation of an embodiment of a 60°-TN liquid crystal cell with aretardation value (dΔn) of 350 nm is plotted verses different appliedvoltages. Referring to FIG. 5, when the applied voltage is even at5V_(rms), the residual retardation is still about 50 nm. Referring toFIGS. 4A and 4B, the azimuthal angles of the eigenmodes 1 and 2 areabout 0.5° larger than the bisector when the residual retardation is 50nm. That is, the azimuthal angles of the eigenmodes 1 and 2 are about30.5° and 120.5°, respectively.

In projection displays, it is desirable to decrease the driving voltagein order to minimize the fringe field effect. Because the azimuthalangles of the two eigenmodes deviate from the direction of the bisector(or the direction perpendicular to the bisector), the polarizingdirection of PBS 7 can be oriented to be parallel or perpendicular tothe azimuthal angles of the eigenmodes of the TN type liquid crystalcell at the desired driving voltage. An example is provided toillustrate a feature of the disclosure. Please refer to FIG. 5. A darkstate with a driving voltage of 3.5V_(rms) has a corresponding residualretardation of about 75 nm. From FIGS. 4A and 4B, it is found that theazimuthal angles of the eigenmodes are about 1.5° greater than bisector(30°/120°) when the residual retardation is 75 nm. Thus, referring toFIG. 2, the included angle β between the polarization direction 71 ofPBS 7 and the first alignment direction 3 of the alignment layer 13 isset at about φ/2+1.5° or π/2+φ/2+1.5°. In such a situation, a perfectdark state occurs at the low driving voltage of about 3.5V_(rms).

From FIGS. 4A, 4B, and 5, a relationship among the applied voltage, theresidual retardation, and the azimuthal angle is obtained. As theapplied voltage increases, the residual retardation decreases so thatthe corresponding azimuthal angles of the two eigenmodes changeaccordingly. According to various embodiments in order to obtain areflective liquid crystal light valve with a lower driving voltage and ahigher contrast ratio, a relationship between the included angle φ andthe included angle β should satisfy φ/2<β<φ/2+30° or 90°+φ/2<β<φ/2+120°.As such, the included angle φ is between the first and second alignmentdirections 3 and 4, the included angle β is between the first alignmentdirection 3 and the polarization direction 71 of PBS 7 and numeral 25denotes the bisector of the included angle φ. According to variousembodiments the included angle φ can be between 40° and 70°. In furtherembodiments, the included angle β can be φ/2+1° to about 3°, and instill further embodiments, φ/2+1.5°. That is, the polarization directionof the polarizing device 7, such as a PBS, disposed on the exterior ofthe transparent panel that provides improved results 1 is not theconventional bisector 25.

Note that, when the cell 100 uses right-handed TN type liquid crystalmolecules, the included angle β satisfies −φ/2>β>−φ/2−30° orπ/2−φ/2>β>π/2−φ/2−30°. For convenience, all angles are based on thefirst alignment direction 3 of the front panel 1 (i.e. the firstalignment layer 13), as shown in FIG. 2, and all counterclockwise anglesare defined to be positive.

The following experimental data are provided for better understanding ofvarious embodiments of a reflective light valve having a lower drivingvoltage and a higher contrast ratio than that of the conventionaltechnology.

FIG. 6A depicts a graphical plot of the relationship between thenormalized reflectance and the applied voltage for an embodiment of aleft-handed 60°-TN liquid crystal cell with retardation (dΔn) of 350 nmat different polarization angles β, according to a first test. FIG. 6Bis a local enlarged view of FIG. 6A in the dark state (i.e. the regionthat reflectance is about 0). In the first test, a green incident light(λ=550 nm) is used to impinge the reflective liquid crystal cell 100shown in FIG. 1A. The solid line denotes the bisector (β=φ/2=30.0°) inFIGS. 6A and 6B.

Because the PBS 7 has a limited extinction ratio (ER) of about 1000:1,the contrast ratio (CR) of the reflective light valve is affected by theextinction ratio of PBS as${{CR} = \frac{1}{\left( {1/{ER}} \right) + R}},$wherein R is the normalized reflectance. For example, when thenormalized reflectance is R=0.00005, the contrast ratio isCR=1/(0.001+0.00005)=952.

Referring to FIG. 6B, an applied voltage of about 5V_(rms) providesR=0.00005 when the bisector (β=30.0°) is used as the polarization angle.According to the first test, because the boundary layers are taken intoconsideration, an angle β of about 31.5° provides an improved result.For example, the driving voltage of the dark state drops to 3.5V_(rms),when β is about 31.5°. As shown by the first test, about 3.5V_(rms)results in the same CR=952. Thus, according to an embodiment, thereflective light valve has lower driving voltage than the conventionaltechnology.

Further, using a driving voltage at 3.5V_(rms) of the conventionaltechnology can only obtain a contrast ratio of CR=1/(0.001+0.0012)=455.In contrast, according to various embodiments described herein, adriving voltage of 3.5V_(rms) obtains a much higher contrast ratio of,for example, 952.

Accordingly, the first test verifies that a polarization angle β ofφ/2+1° to about 3°, and in certain embodiments, φ/2+1.5° isadvantageous. An embodiment of the reflective light valve can thusprovide a high contrast ratio with a low driving voltage, therebyreducing power consumption.

FIG. 7A depicts a graphical plot of the relationship between thenormalized reflectance and the applied voltage for an embodiment of aleft-handed 57°-TN liquid crystal cell with retardation (dΔn) of 350 nmat different polarization angles β, according to a second test. FIG. 7Bis a local enlarged view of FIG. 7A in the dark state (i.e. the regionthat reflectance is about 0). In the second test, a green incident light(λ=550 nm) is used to impinge the reflective liquid crystal cell 100shown in FIG. 1A. The solid line denotes the bisector (β=φ/2=28.5°) inFIGS. 7A and 7B.

Referring to FIG. 7B, an applied voltage of about 4.8V_(rms) providesR=0.0001 when the bisector (β=28.5°) is used as the polarization angle.The contrast ratio of the reflective liquid crystal cell 100 at4.8V_(rms) is CR=1/(0.001+0.0001)=909. According to the second test, anangle β of about 30° provides improved results. For example, the drivingvoltage of the dark state drops to about 3.4V_(rms) when β is about 30°.As shown by the second test, about 3.4V_(rms) results in the sameCR=909. Thus, according to an embodiment, the reflective light valve asdescribed herein has a lower driving voltage than the conventionaltechnology.

FIG. 8A depicts a graphical plot of the relationship between thenormalized reflectance and the applied voltage for an embodiment of aleft-handed 55°-TN liquid crystal cell with retardation (dΔn) of 350 nmat different polarization angles β, according to a third test. FIG. 8Bis a local enlarged view of FIG. 8A in the dark state (i.e. the regionthat reflectance is about 0). In the third test, a green incident light(λ=550 nm) is used to impinge the reflective liquid crystal cell 100shown in FIG. 1A. The solid line denotes the bisector (β=φ/2=27.5°) inFIGS. 8A and 8B.

Referring to FIG. 8B, an applied voltage of about 4.8V_(rms) providesR=0.0001 when the bisector (β=27.5°) is used as the polarization angle.The contrast ratio of the reflective liquid crystal cell 100 at4.8V_(rms) is CR=1/(0.001+0.0001)=909. According to the third test, anangle β of about 29° provides an improved result. For example, thedriving voltage of the dark state drops to about 3.4V_(rms) when β isabout 29°. As shown by the third test, about 3.4V_(rms) to reach thesame CR=909. Thus, according to an embodiment of the reflective lightvalve has a lower driving voltage than the conventional technology.

FIG. 9A depicts a graphical plot of the relationship between thenormalized reflectance and the applied voltage for an embodiment of aleft-handed 50°-TN liquid crystal cell with retardation (dΔn) of 350 nmat different polarization angles β, according to a fourth test. FIG. 9Bis a local enlarged view of FIG. 9A in the dark state (i.e. the regionwith reflectance of about 0). In the fourth test, a green incident light(λ=550 nm) is used to impinge the reflective liquid crystal cell 100shown in FIG. 1A. The solid line denotes the bisector (β=φ/2=25°) inFIGS. 9A and 9B.

Referring to FIG. 9B, an applied voltage of about 5V_(rms) providesR=0.0001 when the bisector (β=25°) is used as the polarization angle.The contrast ratio of the reflective liquid crystal cell 100 at 5V_(rms)is CR=1/(0.001+0.0001)=909. According to the fourth test, an angle β ofabout 26.5° provides an improved result. For example, when β is about26.5°, the driving voltage of the dark state drops to about 3.4V_(rms).As shown by the fourth test about 3.4V_(rms) results in the same CR=909.Thus, according to an embodiment, the reflective light valve asdescribed herein has a lower driving voltage than the conventionaltechnology.

FIG. 10A depicts a graphical plot of the relationship between thenormalized reflectance and the applied voltage for an embodiment of aleft-handed 45°-TN liquid crystal cell with retardation (dΔn) of 355 nmat different polarization angles β, according to a fifth test. FIG. 10Bis a local enlarged view of FIG. 10A in the dark state (i.e. the regionwith reflectance of about 0). In the fifth test, a green incident light(λ=550 nm) is used to impinge the reflective liquid crystal cell 100shown in FIG. 1A. The solid line denotes the bisector (β=φ/2=22.5°) inFIGS. 10A and 10B.

Referring to FIG. 10B, an applied voltage of about 4.7V_(rms) providesR=0.0002 when the bisector (β=22.5°) is used as the polarization angle.The contrast ratio of the reflective liquid crystal cell 100 at4.7V_(rms) is CR=1/(0.001+0.0002)=833. According to the fifth test, anangle β of about 24° provides an improved result. For example, when β isabout 24°, the driving voltage of the dark state drops to about3.4V_(rms). As shown by the fifth test, about 3.4V_(rms) results in thesame CR=833. Thus, according to an embodiment, the reflective lightvalve as described herein has a lower driving voltage than theconventional technology.

FIG. 11A depicts a graphical plot of the relationship between thenormalized reflectance and the applied voltage for an embodiment of aleft-handed 40°-TN liquid crystal cell with retardation (dΔn) of 365 nmat different polarization angles β, according to a sixth test. FIG. 11Bis a local enlarged view of FIG. 11A in the dark state (i.e. the regionwith reflectance of about 0). In FIGS. 11A and 11B, the solid linedenotes the bisector (β=φ/2=20β). Similar to the above tests, when thepolarization angle β is set at φ/2+1.5° (i.e. β=21.5°), an embodiment ofthe reflective light valve of the sixth test can provide a high contrastratio with a lower driving voltage than the conventional technology.

FIG. 12A depicts a graphical plot of the relationship between thenormalized reflectance and the applied voltage for an embodiment of aleft-handed 65°-TN liquid crystal cell with retardation (dΔn) of 345 nmat different polarization angles β, according to a seventh test. FIG.12B is a local enlarged view of FIG. 12A in the dark state (i.e. theregion with reflectance of about 0). In FIGS. 12A and 12B, the solidline denotes the bisector (β=φ/2=32.5°). Similar to the above tests,when the polarization angle β is set at φ/2+1.5° (i.e. β=34°), anembodiment of the reflective light valve of the seventh test can providea high contrast ratio with a lower driving voltage than the conventionaltechnology.

FIG. 13A depicts a graphical plot of the relationship between thenormalized reflectance and the applied voltage for an embodiment of aleft-handed 70°-TN liquid crystal cell with retardation (dΔn) of 345 nmat different polarization angles β, according to an eighth test. FIG.13B is a local enlarged view of FIG. 13A in the dark state (i.e. theregion with reflectance of about 0). In FIGS. 13A and 13B, the solidline denotes the bisector (β=φ/2=35°). Similar to the above tests, whenthe polarization angle β is set at φ/2+1.5° (i.e. β=36.5°), anembodiment of the reflective light valve of the eighth test can providea high contrast ratio with a lower driving voltage than the conventionaltechnology.

An embodiment of a reflective light valve 90 shown in FIG. 1A can becoupled to a controller 142, forming a display device 143 as shown inFIG. 14. The controller 142 can comprise source and gate drivingcircuits (not shown) to control the reflective light valve 90 to renderimages in accordance with an input. The display device 143 andassociated controller 142 may be directed to a reflective projectiondisplay apparatus.

FIG. 15 depicts a schematic diagram illustrating an electronic device151 incorporating an embodiment of the reflective light valve 90. Aninput device 154 is coupled to the controller 142 of the display device143 shown in FIG. 15 to form an electronic device 151. The input device154 can include a processor or the like, inputting data to thecontroller 142 to render an image. The electronic device 151 may be aportable device such as a notebook computer, tablet computer, cellularphone, or a display monitor device, or non-portable device such as adesktop computer or a projection TV.

While the invention has been described by way of example and in terms ofvarious embodiments, it is to be understood that the invention is notlimited thereto. On the contrary, it is intended to cover variousmodifications and similar arrangements as would be apparent to thoseskilled in the art. Therefore, the scope of the appended claims shouldbe accorded the broadest interpretation so as to encompass all suchmodifications and similar arrangements.

1. A reflective light valve, comprising: a transparent substratedisposed opposite a reflective substrate with a twisted nematic typeliquid crystal material interposed therebetween; a first alignment layerwith a first alignment direction disposed on the transparent substrate;a second alignment layer with a second alignment direction disposed onthe reflective substrate, wherein a first included angle φ is betweenthe first and second alignment directions; and a polarizing devicedisposed on an exterior of the transparent substrate to provide anincident beam having a polarization direction, wherein a second includedangle β is between the first alignment direction and the polarizationdirection; wherein a relationship between the first included angle φ andthe second included angle β satisfies φ/2<β<φ/2+30° or90°+φ/2<β<φ/2+120°.
 2. The reflective light valve according to claim 1,wherein the second included angle β is φ/2+1° to about 3°.
 3. Thereflective light valve according to claim 2, wherein the second includedangle β is φ/2+1.5°.
 4. The reflective light valve according to claim 1,wherein the first included angle φ is between 40° and 70°.
 5. Thereflective light valve according to claim 1, wherein the transparentsubstrate is a glass substrate comprising a transparent electrode formedthereon.
 6. The reflective light valve according to claim 5, wherein thetransparent electrode is an indium tin oxide (ITO) or indium zinc oxide(IZO) layer.
 7. The reflective light valve according to claim 1, whereinthe reflective substrate is a silicon substrate comprising a metalelectrode formed thereon.
 8. The reflective light valve according toclaim 7, wherein the metal electrode is an aluminum layer.
 9. Thereflective light valve according to claim 1, wherein the twisted nematictype liquid crystal material comprises positive dielectric anisotropicliquid crystal molecules.
 10. A reflective light valve, comprising: aliquid crystal cell comprising a transparent electrode, a reflectiveelectrode and a twisted nematic liquid crystal layer interposedtherebetween, wherein a retardation value (dΔn) of the twisted nematicliquid crystal layer is about 350 nm; a first alignment layer with afirst alignment direction disposed on the transparent electrode; asecond alignment layer with a second alignment direction disposed on thereflective electrode, wherein a first included angle φ is between thefirst and second alignment directions; and a polarizing device disposedon an exterior of the transparent electrode to provide an incident beamhaving a polarization direction, wherein a second included angle β isbetween the first alignment direction and the polarization direction;wherein a relationship between the first included angle φ and the secondincluded angle β satisfies φ/2<β<φ/2+30° or 90°+φ/2<β<φ/2+120°.
 11. Thereflective light valve according to claim 10, wherein the secondincluded angle β is φ/2+1° to about 3°.
 12. The reflective light valveaccording to claim 11, wherein the second included angle β is φ/2+1.5°.13. The reflective light valve according to claim 10, wherein the firstincluded angle φ is between 40° and 70°.
 14. The reflective light valveaccording to claim 10, wherein the transparent electrode is an ITO orIZO layer and the reflective electrode is an aluminum layer.
 15. Thereflective light valve according to claim 10, wherein the twistednematic type liquid crystal layer comprises positive dielectricanisotropic liquid crystal molecules.
 16. A reflective light valve,comprising: a liquid crystal cell comprising a transparent electrode ona transparent substrate, a reflective electrode on a semiconductorsubstrate and a twisted nematic liquid crystal layer interposedtherebetween; a first alignment layer with a first alignment directiondisposed on the transparent electrode; a second alignment layer with asecond alignment direction disposed on the reflective electrode, whereina first included angle φis between the first and second alignmentdirections; and a polarizing beam splitter disposed on an exterior ofthe transparent substrate to provide an incident beam having apolarization direction, wherein a second included angle β is between thefirst alignment direction and the polarization direction; wherein arelationship between the first included angle φ and the second includedangle β satisfies φ/2<β<φ/2+1°˜3° or 90°+φ/2<β<φ/2+91°˜93°.
 17. Thereflective light valve according to claim 16, wherein the secondincluded angle β is φ/2+1.5°.
 18. The reflective light valve accordingto claim 16, wherein the first included angle φ is between 40° and 70°.19. The reflective light valve according to claim 16, wherein thetwisted nematic type liquid crystal layer comprises positive dielectricanisotropic liquid crystal molecules.
 20. An electronic device,comprising: a reflective light valve of claim 16; a controller coupledto the reflective light valve; and an input device coupled to thecontroller to input data to the controller to render an image.